The optimization of microalgal culturing in liquid digestate after struvite precipitation using gray relational analysis

Abstract

Liquid digestate (LD) is highly turbid and contains ammonium (NH₄⁺–N), which negatively influences microalgal growth. Therefore, a method of reducing LD turbidity and NH₄⁺–N content is proposed, using struvite precipitation. To obtain struvite precipitation supernatant with an ideal UV transmittance, NH₄⁺–N concentration, and N/P ratio for microalgal growth, the effects of pH and the molar ratio of NH₄⁺/Mg²⁺/PO₄³⁻ were studied. Results show that the optimal NH₄⁺/Mg²⁺/PO₄³⁻ molar ratio was 1:1.5:1.5, with a pH value of 8.5, following NaOH addition. Gray relational analysis (GRA) was applied to obtain a relative gray scale for the evaluation of multiple outputs. Results show that Chlorella regularis FACHB-1068 was the optimal microalgae species to support growth in the struvite precipitation supernatant. Using struvite precipitation and treatment with cultured C. regularis FACHB-1068 for 7 days, the removal efficiencies of NH₄⁺–N, PO₄³⁻−P, and COD in LD were 96.52, 99.33, and 35.30%, respectively.